Alternating micro-vias and throughboard vias to create PCB routing channels in BGA interconnect grid

ABSTRACT

A printed circuit board (PCB) assembly having a plurality of circuit layers including outer layers and intervening layers with through-vias and micro-vias used to translate a portion of the signal connections of the grid, thereby creating a set of diagonal routing channels between the vias on internal layers of the board and a BGA package mounted on the printed circuit board.

REFERENCE TO RELATED PATENT APPLICATIONS

The present application is related to the following U.S. patentapplications which are assigned to the assignee hereof:

No. 10/991,360 filed Nov. 19, 2004, inventor Paul Brown entitledOFF-WIDTH PITCH FOR IMPROVED CIRCUIT CARD ROUTING; and

No. 11/041,727 filed Jan. 25, 2005, inventor Alex Chan entitled OFF-GRIDDECOUPLING OF BALL GRID ARRAY (BGA) DEVICES.

BACKGROUND OF THE INVENTION

This invention relates to increasing the efficiency with which signalsfrom area array packages (BGAs and CGAs) are extracted and routed on theprinted circuit boards (PCB or circuit card) on which these devices aremounted.

It is well established that the increase in area array connections (pincount) translates to an increase in the number of printed circuit boardrouting layers needed to support the device escape and routing. Theincentive for further increases in routing efficiency stems from thecost and complications related to the added layers needed for typicalrouting methodologies.

At one extreme, where the area array pin count may be relatively low,implementation of higher efficiency routing methodologies are drivensolely by the cost savings associated with the resulting lower layercount PCB.

As pin count increases above 1000 pins, limitations in high layer countPCB fabrication capabilities, shift the incentive towards minimizing theoverall layer count, thereby improving the quality and reliability of aPCB for a given design, while also allowing the design to benefit fromcost savings.

Finally, at the other extreme, area array technology is following adesign trend towards higher pin counts (>2500) and finer pitches (<0.8mm) which may not be usable at all, without improved routing solutionslike the one proposed.

The prior art has used shared through-vias on power and groundconnections aligned in columns in order to form a routing channel (i.e.a larger space between the row of shared vias and an adjacent row)through which connections can be routed. (See FIG. 1).

The prior art solution places additional constraints on the pinoutconfiguration of the area array device power and ground pins, whichrequire a high degree of adherence to a specific pinout pattern which inthe extreme may not be achievable by certain area array packagingstyles. In general, these added constraints are undesirable from asignal integrity standpoint. Furthermore that solution involves thesharing of adjacent power and ground pins which is also an unfavorablepractice due to the negative impact that it might have on devicefunctionality.

THE PRESENT INVENTION

Simple patterns, created through the intelligent reconfiguration ofprinted circuit board interconnect structures, have been discoveredwhich permit an increase in escape densities that in turn enable therouting of area array devices in fewer layers.

By interspersing micro-via and through-via interconnect, diagonal rowsof contacts can be addressed by interconnect that align on the diagonal.As a result, diagonal routing channels are created on the first andsubsequent internal layer of the printed circuit board. These diagonalchannels have several properties which favor the densification ofrouting: They are wider than orthogonal routing channels, therebyenabling the passage of a greater number of circuit traces.Additionally, the pattern created provides a diagonal set of routingchannels on the second and subsequent layers of the board runningparallel or at right angles to the channels on the first internal layer.These broader routing channels can permit orthogonal routing of adjacentlayers, thereby minimizing the potential for cross talk between signaltraces on these layers. Finally, the micro-vias employed in thisinvention cease to exist on the first internal layer, meaning that halfof the interconnect is eliminated on all subsequent layers, thinning theinterconnect to half of its original density in those areas where thistechnique is employed.

While in theory any pinout configuration can be accommodated by thisinvention, in practice it is best to route power and ground connectionswith through-vias, leaving the micro-vias for the signal carryingconnections. Given that every second via is designated a through-via,the limitation that this preference imposes on a given pinout is notoverly burdensome.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the inventionwill become more apparent when considered with the followingspecification and accompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of a prior art printed circuitboard solution;

FIGS. 2 a, 2 b and 2 c are diagrammatic representations of layers 1, 2and 3, respectively, of a PCB incorporating the invention; and

FIG. 3 a is a diagrammatic illustration of an unaltered area array (BGA)pin field; FIG. 3 b is a diagrammatic illustration of an optimum BGApinout in which preferential signal pins are reserved (R) forcorresponding micro-vias; FIG. 3 c is a diagrammatic illustration offuture channel locations shown corresponding to signal pin locations;and FIG. 3 d is a diagrammatic illustration of remaining pin locationsfor pins requiring through-via interconnects.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, it is a top view of a surface layer of a printedcircuit board (PCB) incorporating one prior art solution. Ball gridarray (BGA) ball contact pads 10 are linked by links 11 to through-boardvias 12. In order to provide a routing channel 20, selected rows of ballcontact pads 15 and 16 are connected through links 17 and 18 to commonthrough-board vias 19 which are designated herein as sharedthrough-board vias. See the above discussion for the disadvantages ofthis solution.

Simple patterns, created through the intelligent reconfiguration ofprinted circuit board interconnect structures have been identified,which permit an increase in escape densities that in turn enable therouting of area array devices in fewer layers.

Referring to FIGS. 2 a, 2 b and 2 c, by interspersing micro-via MV andthrough-via 12′ interconnect, diagonal rows of contacts can be addressedby interconnect that align on the diagonal (see FIG. 2 a). (The elementswith prime numerals correspond to elements in FIG. 1.) As a result,diagonal routing channels DRC are created on the first internal layer ofthe printed circuit board (see FIG. 2 b). These diagonal routingchannels DRC have several properties which favor the densification ofrouting: They are wider than orthogonal routing channels, therebyenabling the passage of a greater number of traces. Additionally, thepattern created provides a diagonal set of routing channels DRC on thesecond and subsequent layers of the board running parallel or at rightangles to the channels on the first internal layer (see FIG. 2 c). Thesebroader routing channels can permit orthogonal routing of adjacentlayers, thereby minimizing the potential for cross talk between signaltraces on these layers. Note that the circuit lines CL-2 in FIG. 2 b mayangle downwardly to the right while the circuit lines CL-3 in FIG. 2 cmay angle downwardly to the left. Finally, the micro-vias MV employed inthis invention cease to exist on the first internal layer, meaning thathalf of the interconnect is eliminated on all subsequent layers,thinning the interconnect to half of its original density in those areaswhere this technique is employed (see FIG. 2 c). (In FIGS. 2 a, 2 b and2 c, fictitious construction lines FCL delineate the original BGA pitchand highlight the effect of interconnect transformation according to theinvention.)

Referring now to FIGS. 3 a-3 d, FIG. 3 a is an illustration of theunaltered area array (BGA) pin field PF. FIG. 3 b illustratesdiagrammatically one optimum BGA pinout in which the preferential signalpins are reserved (R) for corresponding micro-vias shown in FIGS. 2 a, 2b and 3 c. The BGA's package pinout is characterized in that the powerand ground pins are preferably laid out such that they do not fall onalternating contact locations in any given row so that they do not fallon the micro-vias locations as exemplary designated by the R pinlocations. The remaining pin locations shown in FIG. 3 d (with thereserved pin locations not shown for clarity) are pins which requirethrough-via interconnects.

While in theory any pinout configuration can be accommodated by thisinvention, in practice it is best to route power and ground connectionswith through-vias, leaving the micro-vias for the signal carryingconnections. Given that every second via is designated a through-via,the limitation that this preference imposes on a given pinout is notoverly burdensome.

It will be appreciated that a software tool which automates the creationand associated routing of this grid translation method are in thecontemplation of the invention. This software tool could also be used bythe designer of the area array device to achieve the optimum pinout.

The present embodiment has the advantage that it creates effectiverouting channels with minimal detrimental impact to signal integrity orchip decoupling solutions.

Further advantages of the present solution include:

-   1. Provides the capability to route high pin count devices    (i.e. >1200 pins) using available board technology. This ensures    supply while reducing the cost of high complexity substrates by as    much as 50%.-   2. Simplifies chip escape and increases routing flexibility thereby    minimizing design time and by consequence overall time to market.-   3. Given that high pin count devices often unnecessarily drive layer    count above what is needed for the actual design routing. This    solution minimizes this effect, making designs more efficient and    cost effective.

Although in the optimum configuration, this solution puts somelimitations on the placement of power and ground connections, sincetypically half of the positions are still available for power andground, and up to only a third of the connections are typically used forpower and ground, this limitation should not be too onerous.

While the invention has been described in relation to preferredembodiments of the invention, it will be appreciated that otherembodiments, adaptations and modifications of the invention will beapparent to those skilled in the art.

1. A printed circuit board (PCB) having a plurality of circuit layersincluding an outer layer, a first layer and subsequent internal layerswith through-via interconnects and micro-vias in a grid pattern totranslate a portion of the signal connections of the grid pattern,thereby creating one or more diagonal circuit routing channels betweenthe through-vias on at least one subsequent internal layer of the PCB.2. A printed circuit board (PCB) assembly having a plurality of circuitlayers including an outer layer, a first internal layer and subsequentinternal layers with through-via interconnects and micro-vias used totranslate a portion of the signal connections of the grid, therebycreating a set of diagonal routing channels between the vias on internallayers of the board and at least one ball grid array (BGA) packagemounted on said PCB.
 3. The assembly defined in claim 2 wherein BGApackage is characterized in that power and ground connections are laidout such that they do not fall on alternating contact locations in anygiven row or column.
 4. A ball grid array (BGA) package adapted for usewith a layered printed circuit board (PCB) having micro-vias used totranslate a portion of the signal connections of the grid, therebycreating diagonal routing channels between the through-vias on internallayers of the board.
 5. The BGA package defined in claim 4 furthercharacterized in that power and ground connections are laid out suchthat they do not fall on alternating contact locations in any given row.